Incorporating fibrous reinforcements, such as glass fibers and rockwool fibers, into various resins is known to improve dimensional stability, heat distortion temperature, creep resistance, tensile strength and, most dramatically, elastic modulus. However, this always results in a serious deterioration in overall ductility, manifested in poor notched and unnotched impact strength as well as a decreased falling ball impact strength. Even small amounts of fibrous reinforcements have a serious effect on the ductility of polyesters. If it is sought to improve impact performance by adding conventional impact modifiers, such as selectively hydrogenates styrene-butadienestyrene block copolymers, then there is a detrimental effect on stiffness (modulus) and only a minor improvement in impact strength, in any event. It has been found that elimination of the adhesive bond between the polyester and the fibrous reinforcing agent can be accomplished by burning off or otherwise using fibers free of conventional sizing or coupling agents. This does improve ductility, but only for relatively low fiber contents, e.g., up to less than about 10% by weight of sizing-free glass fibers in the polyesters. This is usually below the optimum amount.
It has now been discovered that the addition of poly C.sub.1 -C.sub.10 alkyl (or phenyl) hydrogen siloxanes to compositions comprising "pristine" (or sizing-free) fibrous reinforcements and polyester, in which the fiber content exceeds even 30%, results in a tremendous improvement in falling ball (ductile) impact strength, and notched impact and unnotched impact strengths, too. These can be improved by several hundred percent with almost full retention of the elastic modulus.
It is believed that the following conditions are essential herein:
(i) sizing agents (on the fibrous reinforcement or separately added) must be absent because these either evoke adhesive bonds between the matrix and fiber, or they prevent reactions betwen the hydrogen polysiloxane and the fiber, or both;
(ii) a very good dispersion of the fibers in the matrix is required;
(iii) for best combination of high modulus and creep performance, the addition of polysiloxane is preferably kept below 1.0% and, especially preferably, below 0.5%; and
(iv) the polysiloxane used must contain hydrogen silicon bonds.
Following the use, especially, of short glass fibers, additional advantages in improved isotropy and high surface quality are obtained. It is again reemphasized, that sizing agents must not be present to contribute to adhesive bonds between matrix and fibers, nor should they prevent reactions between the silicon-hyrdogen bond-containing polysiloxane and the fibers. In practical terms this means that pristine fibers should be used. Using the factors mentioned above, the falling dart impact strength of a 20% short glass fiber-reinforced polyesters can be increased from &lt;5J to 80 J, while the unnotched impact bar increases from 350 to 950 J/m. The new composition has a desirable high modulus. These results are evident at surprisingly low levels of hydrogen polysiloxane. Substantially the same results are also obtained with other fibrous fillers, pristine or virgin, including rockwool-mineral fibers, carbon fibers, and the like.